30 THE MARQUETTE lEON-BEAEING DISTIIICT. 



rock is mentioned as containing fragments of slate and jasper, and hence as 

 being younger than these rocks. Granite is said to have intruded tlie 

 cjuartzite of the southern ridge, causing great dislocations in its beds, and 

 metamorphosing it to such an extent as to destroy its bedding planes. 

 Siliceous slates and marbles are interstratihed with the quartzites near 

 the lake. North of the cjuartzite range the country as far north as the 

 Dead River is underlain by chlorite-slates and talcose slates, intersected 

 b)' three east-and-west belts of igneous rocks, many of which ai'e thought 

 to occur as sheets. "Many of the slates appear to be composed of pul- 

 verulent greenstone, as though they might originally have been ejected 

 as an ash and subsequently deposited as a sediment, and pass bv imper- 

 ceptil^le gradations from a highly fissile to a highly compact state" (p. 16). 

 Tliis is the first expression of the view that some of the greenstone-schists 

 of the region were originally volcanic ashes, alth(jugh the illustrations 

 offered are not always of the rocks which were later shown to be tuffs, 

 and the processes by which the ashes were made were not conceived as 

 the same in nature as we now regard them. 



Where the Azoic slates and the overlying Potsdam sandstone are in 

 contact, the latter may plainly be seen to be the young-er rock. At L'Anse 

 (which is at the head of Keweenaw Bay and outside of our district) there 

 is an unconformable superposition of the sandstones and the slates. The 

 authors picture this and describe it in some detail as of great importance. 



This section is esceedius'ly iiisti'uctive, inasmuch as it enables lis to draw a line 

 of demarcation between two formations, ditt'erent in age and external characters. 

 While the newer formation — the Potsdam sandstone — is but slightly if at all disturbed, 

 and little changed by metamorphism, the older or Azoic slates are contorted and folded 

 into numerous arches, and in several places invaded by igneous rocks. Their structure 

 has been changed from granular to subcrystallme, and tlie whole mass is intersected 

 by numerous planes of lamination. (P. ]!).) 



The granite of the Azoic was found to form man}' of the points of the 

 lake north of Little Presque Isle, and to occur inland in rounded, dome-like 

 hills. Everywhere this granite is cut by "powerful" dikes of greenstone 

 and veins of ([uartz. 



In describing a section from the shore of Lake Superior across Teal 

 Lake to the mouth of the Escanaba River, the authors give many details as 



